A. Field of the Invention
The present invention relates generally to electronic components utilizing high-frequency elements and to methods for manufacturing such electronic components. Among the types of electronic component addressed by the present invention are: (1) an electro-acoustic hybrid integrated circuit which uses the conversion of sonic energy into electric energy or vice versa, and more particularly, a high frequency circuit such as a voltage controlled oscillator which incorporates a semiconductor device and an electro-acoustic element such as a surface acoustic wave resonator or a quartz oscillator; (2) a quartz device such as a quartz oscillator or quartz filter for use in, for example data and other communications devices; and (3) a piezoelectric filter using piezoelectric properties of an oscillatory piezoelectric plate made of quartz, lithium tantalate, lithium niobate or lithium borate, for use in, for example, mobile communications and the like.
B. Description of the Related Art
(1) An electro-acoustic circuit such as a voltage controlled oscillator (VCO) comprises a transistor as an active element so as to cause the oscillation, an electro-acoustic element to resonate or oscillate at a desired frequency, as well as some electronics components such as capacitors and resistors. The electro-acoustic element is an element which converts sonic energy into electric energy or vice versa, such as a surface acoustic wave resonator of lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) or lithium borate (Li.sub.2 B.sub.7 O.sub.4) or a quartz oscillator.
An electro-acoustic circuit includes the following circuits. FIG. 1 shows an example of a voltage controlled oscillator which comprises a surface acoustic wave resonator (SAW) as an electro-acoustic element, a transistor (Tr), varactor diodes (D) and passive components of capacitors, inductors and resistors. FIG. 2 shows another example of a voltage controlled oscillator wherein a crystal resonator (X) as an electro-acoustic element is used instead of the surface wave resonator. FIG. 3 shows an example of a high frequency amplifier which comprises a frequency filter (F) of quartz filter or surface acoustic wave filter, a transistor (Tr) and passive components. FIG. 4 shows an example of a temperature compensated crystal oscillator (TCXO) which comprises a quartz oscillator (X), a transistor (Tr), varactor diodes (D), a thermistor (Th) and passive components. That is, these circuits include an electro-acoustic element such as a surface acoustic wave resonator, a quartz oscillator or a surface acoustic wave filter or quartz filter besides a transistor as an active component.
Previously, an electro-acoustic element was sealed in a container such as metal case in order to keep the prescribed oscillation or resonance frequency stable for a long time, as well as an electro-acoustic circuit constructed on a board by mounting various components including the electro-acoustic element thereon. However, this makes the size of the surface acoustic wave resonator or the quartz oscillator several times the size of the relevant resonance or oscillation section itself. Then for applications such as car telephones or portable telephones which are required to be compact, it is an important problem to make the size of an electro-acoustic circuit smaller.
In order to reduce the size of such an electronics circuit, it is desirable to integrate a semiconductor element including a transistor with an electro-acoustic element. For example, K. Tsubouchi et al. produces a surface acoustic wave (SAW) device by growing an aluminum nitride film as a piezoelectric substance on a silicon substrate (Zero Temperature-Coefficient SAW Devices on AlN Epitaxial Films, IEEE Transactions on Sonics and Ultrasonics, SU-32 (1985) 634-644). In order to realize a good oscillation or resonance characteristic, a film has to be grown epitaxially or aligned in a specified crystal axis direction. However, such an epitaxially grown or aligned film is realized only for some materials such as aluminum nitride or zinc oxide by a conventional thin film technique, while a material such as lithium niobate or lithium tantalate suitable for a surface acoustic wave resonator cannot be integrated.
As to a quartz oscillator, it is required to reduce the thickness with polishing or etching in order to produce a high frequency oscillator in the submicrowave band from 800 MHz to 1.9 GHz used for car telephones and for portable telephones. For example, A. Lepek et al. (A New Design for High Frequency Bulk Resonators, 43rd Annual Symposium on frequency Control (1989) pp. 544-547) reported that quartz is polished to a thickness of about 10 .mu.m with a precise polishing technique so as to realize the oscillation at a few hundreds MHz. E. A. Gerber et al. (Advances in Microwave Acoustic Frequency Sources, IEEE Transactions on MTT 34 (1986) 1002-1016) also reported quartz resonators operating above 1 GHz.
However, it is very difficult to manufacture the quartz oscillator or the quartz resonator of a thickness less than 10 .mu.m practically. If the thickness is decreased less than 10 .mu.m, it is difficult to fix the quartz plate as an oscillator because the mechanical strength is very weak and the handling of the plate is difficult. Then, the productivity is poor and the cost is high. Practically, it is very difficult to substantially produce a voltage controlled oscillator at a high frequency of 500 MHz or higher by use of a fundamental oscillation mode of quartz oscillator. If a higher harmonic oscillator mode is used, the Q of the resonance decreases. Then, it is also difficult to produce an oscillator of high and stable performance.
In order to make a voltage controlled oscillator compact and to increase the oscillation frequency at the same time, Grudkowski et al. (Fundamental-mode VHF/UHF Miniature Acoustic Resonators and Filters on Silicon, Appl. Phys. Lett. 37 (1980) 993-995) prepared a ZnO film resonator above an air gap on silicon substrate so as to produce a resonator in the submicrowave band. In this case, the resonator of film thickness of a few .mu.m can be prepared easily, and it is possible to produce a resonator in the submicrowave band. However, the temperature dependence of resonance frequency and the Q of the resonance of a ZnO film are worse than quartz oscillator. Therefore the performance of the resonator is inferior to that of a quartz oscillator.
This situation is a common problem that needed to be solved for various high frequency apparatuses such as a voltage controlled oscillator, a temperature compensation crystal oscillator and a high frequency amplifier with use of an electro-acoustic element.
(2) Quartz devices, such as quartz oscillators and quartz filters, have become an essential component in modern data communications because of their extremely high stability. With the advancement of satellite communications, cellular telephones, and other communications devices, device miniaturization combined with improved performance has become a major developmental objective. This is equally true of quartz devices.
Strict frequency stability is essential to any quartz device. Because the frequency of the quartz device varies with the stress applied to the quartz plate, various measures have been used to reduce the stress applied to the quartz plate. For example, in the case of a chip-type quartz device, the quartz plate is fixed with a conductive adhesive to a metallic holding member having a flexible construction inhibiting the application of stress with the complete assembly sealed in a housing in a vacuum or with an inert gas to maintain long-term frequency stability. However, the frequency stability of this construction as the temperature varies is poor because stress applied to the quartz plate as a result of the difference in the thermal expansion coefficients of the metallic holding member, conductive adhesive, and quartz plate cannot be avoided.
Other problems also arise from the use of a conductive adhesive. First, because the adhesive area must be minimized in order to reduce the affect of the adhesive on quartz oscillation, this construction suffers from low mechanical strength and resistance to drops and impact shock. Because of the low heat resistance of the conductive adhesive, the conditions under which the quartz device can be soldered to a circuit board are extremely limited. In addition, the gases emitted during curing of the conductive adhesive are also emitted after sealing the structure, and contribute to deterioration of long-term frequency stability.
Furthermore, the relationship between the position of the quartz plate and the quartz device housing is limited because a space must be provided for applying the adhesive, and a large gap must be provided between the quartz plate and housing to reduce the change in the oscillation frequency with the generation of thermal stress from the difference in thermal expansion coefficients. Even if the size of the quartz plate itself can be reduced, miniaturization of the complete quartz device (including the housing) is extremely difficult.
A method for holding the quartz plate by means of a quartz holding member has been proposed (Japanese patent laid-open number 1990-291710) for the purpose of alleviating the stress applied to the quartz plate by the difference in thermal expansion coefficients. In this method, the quartz vibration member is held to the substrate by a quartz holding member with the direction securing the base and the quartz holding member perpendicular to the direction securing the quartz holding member and quartz vibration member. This suppresses the frequency-change resulting from stress caused by a change in the temperature applied to the quartz vibration member.
However, even this method cannot ignore the effects of the thermal expansion coefficient of the conductive adhesive securing the quartz vibration member and quartz holding member, and the frequency stability as the temperature changes is insufficient. For this method to be sufficiently effective, the adhesive area of the adhesive and the conductive adhesive must in principle also be small, resulting in problems with adhesive strength. In addition, the problems caused by gases emitted during adhesive curing described above still remain. As a result, this method cannot be considered to be a sufficient solution.
While applied quartz device products include quartz oscillators, temperature correction quartz oscillators (TCXO), and voltage control quartz oscillators (VCXO), miniaturization has also become a major developmental objective for these products. The quartz devices and control circuits driving the quartz devices used in these products are separately manufactured, assembled, and sealed in a housing. As a result, miniaturizing the complete assembly is still difficult.
It is possible to present a one chip type quartz oscillator, TCXO or VCXO, if it is possible to incorporate a quartz plate to the semiconductor substrate having an IC control circuit. However, the frequency stability of the quartz can be obtained only by preparing the quartz in special steps involving predetermined cutting angle and the crystallization of the single crystal quartz. It is not possible to present a thin plate, single crystalline quartz with a desired crystallization direction by applying the quartz according to the conventional deposition or sputtering.
Other than quartz, aluminum nitride (AlN) and zinc oxide (ZnO) are known as thin plate material having piezoelectric properties, the piezoelectric devices using thin plate made by these materials are not appropriate for the use in the communication devices due to poor quality factor Q and frequency stability, when compared with those of the quartz.
Therefore, the frequency stability--temperature characteristic of a conventional quartz device as described above is impaired because the quartz plate is attached by means of metallic members and adhesives having thermal expansion coefficients different from that of the quartz plate. Other problems with the prior art as described above include low mechanical strength and resistance to dropping and impact shock because the quartz plate is attached with adhesive, reliability problems caused by the heat of soldering, and deterioration of long-term frequency stability resulting from gases emitted from the adhesive inside the sealed housing. There are also fundamental problems relating to the miniaturization of the quartz device and applied quartz device products caused by the structure of the quartz device.
(3) In response to development of mobile communications, there is demand for compact and light elements including filters or the like. Conventionally, various filters have been used as a first intermediate frequency filter for mobile communications and mainly include a piezoelectric filter and an elastic surface wave filter. Generally, the piezoelectric filter has such a basic construction as shown in FIG. 58 and includes an oscillatory piezoelectric plate 1501, a pair of upper electrodes 1502 formed on an upper face of the piezoelectric plate 1501 and a lower electrode 1503 formed on a lower face of the piezoelectric plate 1501. The upper electrodes 1502 and the lower electrode 1503 on the single piezoelectric plate 1501 form two sets of counter electrodes. This known piezoelectric filter is a double mode monolithic piezoelectric filter based on a principle in which electric signals are converted into mechanical oscillations by one set of the counter electrodes and the mechanical oscillations are converted into the electric signals by the other set of the counter electrodes. Bandwidth obtained in such a filter depends upon piezoelectric constant of the piezoelectric plate 1501 and is about 0.2% of that at a central frequency in fundamental mode when an AT-CUT quartz plate in ordinary use is employed. In the case of higher-order mode, i.e. overtone of n-th order, bandwidth is further narrowed to 1/(n.sup.2) of 0.2%.
Meanwhile, in response to recent digitization of mobile communications, channel bands have been widened to about 300 KHz. Meanwhile, first intermediate frequency is raised to about 200 MHz by widening of the channel bands. In the known high-frequency piezoelectric filter, since overtone of higher order is employed, it is difficult to obtain a broad band. Consequently, generally, the elastic surface wave filter has been used for the broad-band first intermediate frequency filter. However, the elastic surface wave filter is neither satisfactory in both shape and weight nor is electrically sufficient due to insertion loss, etc. Meanwhile, in order to operate the electrically excellent piezoelectric filter in fundamental wave mode, the quartz plate should be made thin because the resonance frequency is inversely proportional to thickness of the quartz plate. When the AT-CUT quartz plate in ordinary use is oscillated at, for example, 100 MHz, the quartz plate should have a thickness of about 17 .mu.m and thus, production of the quartz plate is extremely difficult. Furthermore, even if such a quartz plate is produced satisfactorily, handling for mounting the quartz plate and connection of the quartz plate to an external circuit are quite difficult due to small thickness of the quartz plate.
Therefore, for example, a method is also proposed by Japanese Patent Laid-Open Publication No. 3-235408 (1991) in which quartz of an oscillatory portion is made thin by etching so as to be capable of being used at high frequency. In this known method, only the oscillatory portion of the quartz plate having a thickness of about 70 .mu.m is made thin to about 20 .mu.m by etching such that high-frequency oscillation of the oscillatory portion is made possible. Since resonance frequency is controlled by etching, it will be extremely difficult due partly to etching accuracy to mass produce filters in which bandwidth is quite narrow and accuracy of central frequency is strict.
Meanwhile, in order to widen the band, electro-mechanical coupling coefficient of the piezoelectric substrate may be changed to a larger one. Nevertheless, since there is not much difference of velocity of sound therebetween, problem of thickness control is not solved yet.
A piezoelectric filter at high frequency is disclosed in, for example, a paper entitled "Film Bulk Acoustic wave Resonator Technology" in 1990 Ultrasonic Symposium Proceedings, page 529 and U.S. Pat. No. 4,719,383 entitled "Piezoelectric Shear Wave Resonator and Method of Making Same". In the known piezoelectric filter, a cushioning film 2102 of SiO.sub.2 is formed on a substrate 2101 of silicon or gallium-arsenic as shown in FIG. 59. Furthermore, after a thin film 2103 of aluminum nitride or zinc oxide has been formed on the cushioning film 2102, electrodes 2104 and 2105 are formed. As a result, a resonator or a filter is obtained. However, in this technology, zinc oxide or aluminum nitride is used for the piezoelectric member. This piezoelectric member can be advantageously formed by sputtering, i.e. thin film forming technology. However, since the piezoelectric member is made of polycrystalline material, crystals should be aligned in C-axis in order to obtain piezoelectric property. Undesirably, this alignment largely depends upon thin film forming conditions or devices and changes according to thickness of the film deposited on the substrate or material of the ground. On the other hand, quartz, lithium niobate, lithium tantalate, lithium borate or the like in polycrystalline form does not exhibit sufficient piezoelectric property. Especially, quartz is widely used for an oscillator, a filter or the like due to its high stability and excellent temperature characteristics but only .varies. quartz which is monocrystalline and has a crystalline structure symmetric with respect to a threefold axis exhibits piezoelectric property.
Meanwhile, a similar technology is disclosed in U.S. Pat. No. 5,036,241 entitled "Piezoelectric Laminate and Method of Manufacture". In the known technology, substance having piezoelectric property is bonded to an insulating member by using an adhesive layer and resistivity of the insulating member is controlled by using temperature or light such that polarization is caused by applying voltage to the insulating member. According to this known method, since accuracy of the piezoelectric member in a direction of its thickness determines accuracy of resonance frequency in a device employing resonance in the direction of thickness of the piezoelectric member, thickness of the adhesive layer is required to be controlled accurately. However, in the case where the AT-CUT quartz substrate acts as the piezoelectric member and its central frequency is set to 100 MHz, the substrate has a thickness of 17 .mu.m. In view of production cost of the piezoelectric filter from mass production efficiency and operation for adjustments, the piezoelectric member should have an accuracy of not more than 1 .mu.m. Since error of thickness of the adhesive layer directly determines accuracy of frequency, it is considered impossible to attain such an accuracy of the piezoelectric member in this method and the arrangement.